Method and apparatus for sensing substances by analysis of adsorbed matter associated with atmospheric particulates

ABSTRACT

A method and apparatus for sensing substances, such as mineral deposits, in which atmospheric particulates in an area being investigated are collected, and adsorbed matter is removed from the particulates. The adsorbed matter (usually in the gaseous state) is then analyzed to provide information concerning the possible existence of the substance in the area being investigated.

United States Patent Barring r 1 1 Oct. 30, 1973 54] METHOD AND APPARATUS FOR SENSING 3,309,518 3/1967 Weiss 73/28 x SUBSTANCES BY ANALYSIS OF 'i ggf p 73/28 0 1n et a ADSORBED MATTER ASSOCIATED WITH 2,756,585 7 1956 lrby 73 19 ATMOSPHERIC PARTICULATES 3,205,700 9/1965 Lively et al. 7.3 19

Inventor: Anthony Rene Barringcr,

Willowdale, Ontario, Canada Assignee: Barringer Research Limited,

Ontario, Canada Filed: May 14, 1971 Appl. No.: 143,671

Foreign Application Priority Data Dec. 8, 1970 Canada 100078 US. Cl 73/28, 23/230 EP, 55/2, 55/133 Int. Cl. G0ln 31/06 0 Field of Search 73/19, 28, 170 R, 73/421.5 R, 421.5 A,53;55/2,131, 133, 55/136, 137, 138, 270; 23/230 EP References Cited UNITED STATES PATENTS Teel et al 73/28 FOREIGN PATENTS OR APPLICATIONS 198,038 6/1967 U.S.S.R ..73/42l.5 1,020,003 11/1957 Germany ..55/133 Primary Examiner-Herbert Goldstein Attorney-Rogers, Bereskin & Parr [57] ABSTRACT A method and apparatus for sensing substances, such as mineral deposits, in which atmospheric particulates in an area being investigated are collected, and adsorbed matter is removed from the particulates. The

adsorbed matter (usually in the gaseous state) is then analyzed to provide information concerning the possible existence of the substance in the area being investigated.

5 Claims, 3 Drawing Figures A/vA L z/A/G A PPA RA 70s PATENIED [1U 30 1973 SHEET 10F 2 FIG. I

A/VALYZ/A/G T0 ANALYZING APPARA rus A PPA RA TL S LIYIT If??? INVENTOR.

ANTHONY RENE BARRINGER PAIENIEUDM 30 ms SHEET 2 BF 2 I NVENTOR. ANTHONY RENE BARRINGER METHOD AND APPARATUS FOR SENSING SUBSTANCES BY ANALYSIS OF ADSORBED MATTER ASSOCIATED WITH ATMOSPHERIC PARTICULATES This invention relates to a method and apparatus for detecting volatile components in the atmosphere for the purpose of identifying mineral deposits. It has been found that a number of volatile components are present at an unusual concentration in the atmosphere overlying certain classes of mineral deposit. Thus, for example, mercury vapour increases from typical backgrounds of one to ten nanog'rams per cubic meter to concentrationsvarying between and '50 nanograms per cubic meter over gold and silver deposits as well as over many types of base metal orebody. This is attributed'to the fact that mercury tends to concentrate in most metallic ores to an amount which is greater than the surrounding rocks and since mercury and some of its compounds have an appreciable vapour pressure at normal atmospheric temperatures, some of this mer-' cury is dispersed into the overlying atmosphere above orebodies. Similarly, there is a geochemical association of chlorine, fluorine, bromine and iodine with many classes of mineral deposit, and since a number of theseelements and their compounds have a significant vapour pressure, these elements and their compounds can also occur in the atmosphere over mineral deposits. In the case of the halogen elements, there is also a strong association with phosphate deposits and oil fields so that these elements can also be used to prospect for oil and phosphates. In addition to these elements there are a number of others that have much lower vapour pressures, but nevertheless can occur in minute but measureable traces in the atmosphere. These include the halide compounds of elements such as tell urium and selenium which have close affinity with certain types of copper deposit, and the halide compounds of elements such as arsenic, antimony and bismuth, all of which have sufficient vapour pressure to be present in minute traces. Given sufficient sensitivity in an analytical system, even more elements can be detected in the form of volatile inorganic compounds. In general, the halides of most metals are substantially more volatiletha'n the metals themselves and can occur in minute quantities in the atmosphere.

In addition to the occurrence in the atmosphere of volatile inorganic compounds there are also present significant amounts of organic vapours. Organic vapours are generated by living forms of all types including plants, soil bacteria, insects and animals. Large quantities of volatile organic vapours are generated by trees in the form of natural oils known as turpines. Some of these organic vapours oxidize in the atmo sphere to form minute liquid and solid particulate matter, a fact which can cause the development of atmospheric haze over forests under some weather conditions. A fractional component of organic vapours in the atmosphere is comprised -of metallo-organic com pounds. It has been shown that certain metals that are present in soils are converted into volatile organic compounds by bacteria. A well established example is that of mercury which is converted into di-methyl mercury by a variety of bacteria and other micro-organisms in soils. Di-methyl mercury has'a lower boiling point than water and readily evaporates into the atmosphere as it is formed. It has been demonstrated that soils containing living micro-organisms liberate substantially more mercury into the atmosphere than the same soils that have been sterilized.

In addition to mercury, there are a number of other metals that can be converted by micro-organisms into volatile metallo-organic compounds which provides an important mechanism for introducing metallic vapours into the atmosphere. The problem in measuring all these components both organic and inorganic is to achieve sufficient sensitivity to be able to make rapid measurements during mobile traversing.,U.S. Pat. No.

3,309,518 of O. Weiss, dated Mar. 14, 1967 disclosesa method of aerial prospecting in which solid particu lates are gathered on a piece of filter paper, following which the filter paper is dissolved, the particulates are concentrated, and then the particulates are analyzed by an electron probe analyzer which ex'citesX-ray emissions from the particulates. By analyzing the X-rays, information is said to be obtained concerning the identity of the elements present in the particulates. Instead of analyzing the particulates themselves for the presence of metals, the present invention treats the particulates as adsorbers of volatile components which are indica tors of the presence of mineral deposits. Particulates are normally present in the atmosphere to a mass concentration of at least 10 micrograms per cubic meter, andare frequently up in the range of several hundred micrograms per cubic meter. It has been shown that volatile components in the atmosphere are adsorbed by particulates and reach an equilibrium. A good example is. that of iodine vapour. It has been demonstrated that free iodine vapour can be percent adsorbed onto natural particulates in the atmosphere in a short period of time. The high adsorption is a function of the very large surface area of particulates in the atmosphere in relation to their mass, sinceparticle sizes extend below one-tenth of a micron and particle concentrations run in the vicinity of thousands of particles per cubic centimeter. Nearly all surfaces are natural adsorbers for gases and vapours and the ability of particulates to scavenge traces of condensable gases and vapours from the atmosphere is a function of this phenomena and their large ratio of surface area to mass.

Metallo-organic vapours'are condensable and susceptible to adsorption by particulate matter and particularly by organicliquld particulates. Thus over heavily forested areas high concentrations of organic liquid particulates may occur which can adsorb the metalloorganic vapours generated by micro-organisms in the highly active and humus rich forest. soils. Thus the volatile components adsorbed on particulates in the atmosphere may be indicators of anomalous metal concentrations in the underlying terrain in vegetated areas as well as in the more obvious arid regions where vegetation is scarce and conditions are dusty.

In the present invention, atmospheric particulates are collected and subsequently heated to drive off adsorbed; gases and vapours, which are then analyzed. The particulates may be collected continuously from a moving aircraft, for example, and concentrated if necessary. A number of analytical techniques may be employed for the purpose of identifying the adsorbed gases and vapours.

As used herein, the word "particulates refers to minute solid or liquid particles in suspension in a gas, particularly the atmosphere. For convenience, the word gas will usually be used to refer to either gases or vapours or both gases and vapours. Adsorbed matter" refers to atoms, molecules, or ions of a solid, liquid or gas held in contact with surfaces or interfaces of finely divided particulates.

FIG. 1 is a diagrammatic sectional view of apparatus according to the invention;

FIG. 2 is a diagrammatic perspective view of the apparatus according to FIG. 1, partly broken away; and

FIG. 3 is a diagrammatic perspective view of a modified form of a cyclone for concentrating solid particulates.

The drawings show an embodiment of the invention in which large volumes of air are sampled through an electrostatic precipitator. The air enters through a sampling duct 1, goes through an electrostatic charging grid 2 and then through a grounded collecting grid 3. The charging grid 2 may consist of a set of grounded wires alternating with a set of wires connected to a source of negative high voltage. The grounded collecting grid 3 is made of nichrome resistance wire which can be heated to a temperature of up to l,000C by the application of a current through the wires. The alr stream is split by a perforated baffle 4 which can be rotated into an alternative position 5 about a pivot point 6. The perforations are arranged in the baffle such that a small percentage of the air stream passes through the perforationsv and the remainder is diverted into the other half of the duct. The collecting grid 3 is split into two portions which can be heated independently and are both kept at ground potential so that at all times they will collect and retain particles. A second unperforated baffle 7 is arranged to swing about pivot point 8 and can be rotated to point 9. Two butterfly valves 10 and 11 can be operated to open and close exit tubes 13a, 13b.

The particulates in the air entering at l are charged on the grid 2 and collected on the grid 3, the air substantially devoid of its particulates then being expelled out of the duct at 12. With the baffles in the position shown in FIG. 1, a small portion of the air passes over the righthand side of the grid 3 and a heating current is applied to this right hand side. Particulates adhering to the right hand side of the grid 3 are raised to a sufficiently high temperature to drive off adsorbed gases and volatile components which then pass out through the open valve 11 along the right hand exit tube 13b. The baffle 7 is closed in order to seal off this small flow of air and channel it out through the tube 13b. At the same time the main body of air passes over the cold left hand portion of the collection grid 3 and particulates are collected. After a short period (e.g., l to 5 seconds more or less), the perforated baffle 4 is swung to the position 5, the unperforated baffle 7 is swung to position 9, the valve 10 is opened and'the valve 11 is closed, and heat is applied to the left hand side of the collection grid3 and removed from the right hand side. The particulates collected on the left hand side of the collection grid 3 are now heated to a temperature sufficient to drive off adsorbed gases and volatile components and a small air stream through perforated baffle 4 carries them through the exit valve 10 and out to the tube 13a. Also, the heating current in the right hand side of the collection grid 3 is switched off and particulates are collected on'this portion of the grid 3 from the main body of the air stream. The cycle is then repeated.

Appropriate control means is provided to control the functions described. The details of these control means are wellwithin the skill of those skilled in the art, and the control means is therefore shown in block form. Typically the control means includes a control box 14 having shafts l5, 15 connected to the baffles 4, 7 at pivot points 6, 8 to swing the baffles in unison from one side of the device to the other, to close off first one grid containing chamber and then the other. Electric valve actuators 17, 18 are connected to the butterfly valves 10, 11 and are connected by electrical leads (not shown) to 'the control box 14 and are controlled thereby so that the appropriate exit tube 13a or 13b will be connected to the device. Control leads 19, 20 also extend from the respective halves of the grid 3 to the control box so that each half grid will be heated when the chamber in which it is located is closed.

Although only two chambers, each containing half of the collection grid 3, have been shown, additional collection chambers can be usedif desired. Alternative means may also be used to collect and heat the particulates, but desirably a continuous record should be provided as the air over the terrain under investigation is traversed.

In this fashion a continuous air stream flows through the exit tube 13 containing a concentration of the volatile components that were originally present in adsorbed form in the incoming air stream. The rate of flow through the perforated baffle4 can be arranged to be very small such as 1 percent of the total incoming air or less. This provides a high degree of concentration. Furthermore, in a simple modification of the system, the perforations in the baffle 4 can be entirely removed and the closed chamber so formed when the baffles are in position can be flushed out with argon from a gas cylinder if so desired. This can provide an inert carrier gas through the heated particulates instead of using oxygen. This can be particularly convenient for some types of emission spectrographic analysis of the vapours. Since the baked particulates accumulate the collection grid 3 must occasionally be cleaned or replaced to maintain efficient operation.

A variety of analytical techniques can be used for analyzing the gases and vapours emerging from the exit tube 13. For example the gases and vapours can be scrubbed with a water spray and the liquid solutions so obtained can be passed over a specific ion electrode. Electrodes are available which can have high sensitivity for the halogen elements such as fluorine and can provide a continuous electrical reading of fluorine concentration. In the case of measuring for mercury vapour, the vapours can be passed through an optical cell and the absorption of a 2537A beam of light can be measured. This absorption is related to the concentration of mercury vapour present. In a more sophisticated analytical arrangement, an argon carrier gas can be used as described above and the gas stream can be passed through a microwave cavity in order to generate a microwave plasma. The emission light from this plasma can be passed into a spectrometer and measurements made of specific emission lines corresponding to elements such as mercury, fluorine, iodine, bromine, chlorine tellurium, arsenic, antimony, bismuth, etc. Since various kinds of analytical apparatus may be used, the

7 ute of air. This represents a cubic meter every 6 seconds which will carry 10 micrograms or more of particulates through the system every 6 seconds. Plasma analytical techniques are capable of achieving sensitivities typically of the order of 10" grams for a wide range of elements so that such a system is able to see as little as 10 grams of an element adsorbed in gas or vapour form on lO grams of solid material. This represents an absorption of 1 ppm byweight. In some cases, sensitivities for elements can be achieved as high as 10' grams (e.g., cadmium). In these cases l0 grams of, for example, cadmium compounded as a halide ororganic vapour and dispersed in adsorbed form through a cubic meter of air can be detected. The utility of such a device for detecting minute dispersions of vapours above mineral deposits is readily apparent. Normalization against the atmospheric particulate concentration and size can be carried out by using conventional equipment such as an optical monitor or dust concentrator which is connected continuously on stream during survey. r

An alternative application of the invention is in the detection of minute traces of hydrocarbons and other organic volatiles in connection with oil exploration. In this case, a similar technique is used except that the electrostatic collecting grid is heated to a lower temperature which does not pyrolise and destroy the organic compounds. A typical temperature is 200C. The output pipe of the instrument l3'is now taken to a gas chromatograph for analysis or to a silicone rubber membrane. The membrane is used to seal off a low pressure zone in front of the entrance port of a mass spectrometer. Organic vapours have the property of passing rapidly in one or twoseconds through such a membrane leaving behind the accompanying molecules of air or other carrier gas; Rapid real-time analysis can then be carried out in the mass spectrometer which may conveniently be of a lightweight type. A suitable instrument is an RF Quadrapole mass spectrometer. Such a system can achieve extreme sensitivity and specificity for organic compounds. It has great utility in oil exploration where the presence of sub-surface oil fields can be indicated by hydrocarbon seeps at the surface. Such seeps can be gaseous and contain methane, ethane, pentane, etc. or they may involve liquid hydrocarbons such as benzene which can have sufficient vapour pressure to escapeinto the atmosphere.

The embodiment of the invention described and shown uses heat for removing volatile components from the particulates. It will be appreciated, however, that theequipment can be modified in order to remove absorbed gases by reduction in pressure. Thus the chamber that is sealed during the heating cycle can be pumped down to low pressure instead, in order to cause degassing of the particulates. Alternatively, a combination of heat and low pressure can be used in order'to minimize heating and obtain a liberation of organic compounds without altering their chemical structure.

In another variant of the invention, adsorbed gases and vapours can be removed by flushing the collected particulates with a gas or liquid that has complexing capabilities for the vapour in question. Thus, certain adsorbed metallic vapours can be removed by passing chlorine over the particulates. The bonding forces of adsorption is. this case are overcome by the stronger bonding force between the chlorine and the metal. The chlorine or other carrier agent can then be analyzed for adsorbed components leached from the particulates.

Although the invention has been described with ref erence to an electrostatic method of particle collection, other techniques may be employed, such as the use of a cyclone separator. A modified type of cyclone suitable for concentrating dust from a large volume of air is shown in FIG. 3, In FIG. 3, incoming air containing particulates enters a cone shaped cyclone 22 through a duct 23 which directs the flow of air'tangentially into the cyclone 22. The air swirls around a perforated cone shaped separator 24, and much of the air escapes through the openings in the separator 24 to a discharge duct 25 having a flow balance valve 26. The particulates tend to settle towards the bottom of the cyclone 22, and thus a stream of air that is enriched in particulates is caused to flow through a discharge duct 27. A still further method is the use of filters of the paper or fibreglass type. The particles frorn'large volumes of air can be collected on filters and the adsorbed gases or volatile components subsequently removed by heating, gas or liquid leaching or vacuum degassing. This can be operated on a semi-continuous real-time basis in the fashion of the electrostatic embodiment already described in detail, or the filter can be stored for later analysis. Thus, for exploration purposes, a continuous strip filter can be used adapted from a standard and commercially available continuous pollution sampler and provide a flight record of particulate matter. Special equipment can be constructed to provide subsequent analysis of the adsorbed gases and volatiles held on the particulates. Correlation can then be made between the analytical data derived from the particulates and the locations which the said particulates were collected.

As in the case of any geophysical prospecting apparatus, it is necessary to correlate the data obtained from the particulates with the locations at which the particulates were obtained. This may be accomplished in the case of an airborne instrument by providing -a strip camera to record photographically the flight lines of the aircraft.

Whereas applications of the invention that have been described relate particularly to mineral and oil exploration, in general the invention is applicable to the solution of problems involving the detection of minute quantities of organic or inorganic vapours. The detection of the presence of concealed narcotics is one example. Many narcotics have sufficient vapour pressures for them to be detectable at some distance by dogs. The

present invention can be used to sample large volumes b. continuously concentrating said particulates immediatelyafter they have been received,

c. removing at least a portion of said adsorbed matter from said particulates immediately after they have been received and concentrated as aforesaid,

. forming a stream of said removed matter,

e. analyzing said removed matter continuously as said matter is removed from said particulates while continuously receiving and concentrating said particulates to determine the identity of said portion of removed matter and hence to ascertain whether or not said substance is present in said area, and

f. said steps (a), (b), (c), (d), and (e) being performed continuously, rapidly and in quick succession.

2. A method as claimed in claim 1 wherein said area is an area of the earth, wherein said substance is indicative of underlying geological conditions, wherein said method is practised from an aircraft and wherein said method includes the additional step of correlating said analyzed matter with said known locations.

3. A method as claimed in claim 2 wherein said matter is removed from said particulates by heating said particulates to a temperature sufficient to drive off said matter and wherein said particulates are transferred to an inert carrier gas prior to the analysis thereof.

4. Apparatus for performing a rapid geochemical survey of an area of the earth from a moving vehicle comprising:

a. means for receiving an air stream containing atmospheric particulates at known locations in said area, which particulates may contain adsorbed matter that is indicative of underlying geological conditions,

b. means for concentrating said particulates immediately after they have been received, said concentrating means being operative continuously whereby immediately after the particulates are received they are concentrated,

c. means connected to said concentrating means for removing at least a portion of said adsorbed matter from said particulates immediately after they have been received and concentrated as aforesaid, said removing means being operative continuously whereby said adsorbed matter is removed from said particulates continuously immediately after said particulates have been concentrated as aforesaid, and

d. means connected to said removing means for analyzing said removed matter continuously as said matter is removed from said particulates to determine the identity of said portion of removed matter and hence to ascertain whether or not said substance is present in said area.

5. Apparatus as claimed in claim 4 wherein said concentrating means includes at least two chambers, each for collecting said particulates, and means for diverting at least a portion of said air stream cyclically between said chambers, so that particulates are collected cyclically in one of said chambers and then in the other, wherein said removing means includes means for heating at least some of the collected particulates to remove matter therefrom in gaseous form, wherein there is further provided means for operating the heating means in said chambers cyclically so that while particulates are collected in one chamber, the concentrated particulates in the other chamber are being heated, and connecting means for connecting said chambers to said analyzing means. 

1. A method of identifying the presence of a substance in an area, comprising: a. receiving an air stream containing atmospheric particulates at known locations in said area, which particulates may comprise adsorbed matter that is indicative of the presence of said substance in said area, b. continuously concentrating said particulates immediately after they have been received, c. removing at least a portion of said adsorbed matter from said particulates immediately after they have been received and concentrated as aforesaid, d. forming a stream of said removed matter, e. analyzing said removed matter continuously as said matter is removed from said particulates while continuously receiving and concentrating said particulates to determine the identity of said portion of removed matter and hence to ascertain whether or not said substance is present in said area, and f. said steps (a), (b), (c), (d), and (e) being performed continuously, rapidly and in quick succession.
 2. A method as claimed in claim 1 wherein said area is an area of the earth, wherein said substance is indicative of underlying geological conditions, wherein said method is practised from an aircraft and wherein said method includes the additional step of correlating said analyzed matter with said known Locations.
 3. A method as claimed in claim 2 wherein said matter is removed from said particulates by heating said particulates to a temperature sufficient to drive off said matter and wherein said particulates are transferred to an inert carrier gas prior to the analysis thereof.
 4. Apparatus for performing a rapid geochemical survey of an area of the earth from a moving vehicle comprising: a. means for receiving an air stream containing atmospheric particulates at known locations in said area, which particulates may contain adsorbed matter that is indicative of underlying geological conditions, b. means for concentrating said particulates immediately after they have been received, said concentrating means being operative continuously whereby immediately after the particulates are received they are concentrated, c. means connected to said concentrating means for removing at least a portion of said adsorbed matter from said particulates immediately after they have been received and concentrated as aforesaid, said removing means being operative continuously whereby said adsorbed matter is removed from said particulates continuously immediately after said particulates have been concentrated as aforesaid, and d. means connected to said removing means for analyzing said removed matter continuously as said matter is removed from said particulates to determine the identity of said portion of removed matter and hence to ascertain whether or not said substance is present in said area.
 5. Apparatus as claimed in claim 4 wherein said concentrating means includes at least two chambers, each for collecting said particulates, and means for diverting at least a portion of said air stream cyclically between said chambers, so that particulates are collected cyclically in one of said chambers and then in the other, wherein said removing means includes means for heating at least some of the collected particulates to remove matter therefrom in gaseous form, wherein there is further provided means for operating the heating means in said chambers cyclically so that while particulates are collected in one chamber, the concentrated particulates in the other chamber are being heated, and connecting means for connecting said chambers to said analyzing means. 